Valve has started to censor messages on the Steam chat system where the message includes links to torrent sites according to Torrent Freak.
It has been noted that links to KickassTorrents gets dumped in the services instant chat system preventing the links being shared.
TorrentFreak ran tests on Steam to figure out what was happening as Valve have remained tight lipped about the addition of the filter. They found that KickassTorrents links and even messages containing just the name gets dumped into the black hole of censorship. In regards to other sites, they currently go through the chat system but are flagged as being potentially malicious.